This application claims the priority of German Patent Document No. 199 60 797.4, filed Dec. 16, 1999, and PCT International Application No. PCT/DE00/04422, filed Dec. 13, 2000, the disclosures of which are expressly incorporated by reference herein.
The invention relates to a method of producing an aperture in a metallic component, in which the aperture comprises, at least in certain portions, a non-cylindrically formed funnel, extends from a first surface to a second surface of the component and is formed by a laser beam.
EP-A-0950463 discloses a method of forming a cooling hole comprising a diffuser portion, in which the diffuser portion is cut out by a beam-drilling method in such a way that the drilling beam remains within a previously selected cross-sectional area.
It is known from S. NOLTE, G. KAMLAGE: xe2x80x98Mikrostrukturierung mit Femtosekundenlasernxe2x80x99 [Microstructuring with femtosecond lasers], LASEROPTO, Vol. 31, No. 3 Apr. 15, 1999 (1999-04-15), pages 72-76, XP000999012, that, during laser machining, an increasingly shorter pulse duration and a reduction in the ambient pressure assist the sublimation of the material and lead to an improved workpiece surface. In particular during laser-beam drilling, the striation of the hole and the drilling core can be improved by a reduction in the ambient pressure and by special process gases, such as helium.
U.S. Pat. No. 5,609,779 discloses a method of laser drilling non-circular apertures in a metallic component, in which the aperture comprises a diffuser which extends up to one surface of the component and is formed by vaporizing the metal by means of a laser, the laser beam traversing the surface from a center line of the diffuser transversely to both sides at a respectively increasing rate and with overlapping laser spots, to allow non-circular apertures to be produced with a conventional laser as inexpensively as possible and with a relatively good surface. The increasing rate and the overlapping are intended to compensate for tolerances in the pulse energy which lead to varying material removal, and at the same time produce the specific, cross-sectionally non-circular form of the diffuser.
In this operation, it has proven to be problematical that the metal becomes liquid due to the pulse frequency and pulse duration used, which has adverse effects on the surface of the diffuser. Owing to the laser machining at an increasing rate, the thickness of the layers varies and decreases from the center line outwards, whereby cumulative inaccuracies with regard to the form of the diffuser may occur, in particular in the case of a number of successive layers.
The object of the present invention is to provide a method of the generic type described at the beginning with which apertures can be formed with the best possible surface and dimensional accuracy.
The solution to the problem is characterized according to the invention in that, by choice of the laser parameters, such as for example the pulse frequency, pulse energy or pulse duration, the metallic material is predominantly removed by sublimation, at least during the formation of the funnel, and the funnel is formed by laser removal of layers of a substantially constant thickness.
It is advantageous in the case of material removal by sublimation, as a result of the high energy input per pulse, that undefined deposits of viscous material in the region of the aperture or the funnel to be removed, which lead to increased roughness, are avoided. The defined removal in layers of substantially constant thickness ensures high dimensional accuracy with low roughness of the surface of the aperture and/or of the funnel. The method can be carried out efficiently from technical production-related aspects. Final finishing of the surface of the aperture or the funnel formed in this way is not required, saving time and costs in the production process.
During the removal of the layer, the laser beam can be moved at a constant rate in relation to the respective surface, beginning with an outer surface, of the component, in order in this way to achieve a defined removal of layers of substantially constant thickness. The relative movement between the laser beam and the component to be machined is generally produced by moving the component, which is clamped in a device of a suitable machine tool. Similarly, this can be achieved by a generally more restricted movement of the laser or a superimposed movement.
Suitable laser parameters allow layers of substantially constant thickness of from 1 xcexcm to 10 xcexcm to be removed. The form and dimensions of the successively removed layers can be adapted to the form of the funnel, whereby the apertures can be produced economically without any finishing. The form of the funnel is substantially described by a first aperture angle, determining a height H of the funnel, and a second aperture angle, determining a width B. The funnel may alternatively also be conically formed with a circular cross section.
For removing the layers, a laser beam can be moved in a number of neighboring, generally parallel, paths over the respective surface of the component, the removal beginning at an outer surface of the metallic component and taking place, for example, line by line for each layer. An overlapping of the individual paths is not required and is avoided to carry out the method as efficiently as possible.
To realize layers of substantially constant thickness, the laser beam can move along the individual paths at a constant rate.
The laser removal can be carried out at a pulse frequency of from 1 to 50 kHz, in order to remove the material predominantly by sublimation during the forming of the aperture and/or the funnel. For this purpose, as much energy per pulse as possible must be introduced into the region to be removed over an extremely short period of time. To realize the extremely short period of time, the laser removal can be carried out with a pulse duration of from 10 to 1000 ns and a pulse energy of 0.005 to 1 Joule per pulse, these laser parameters allowing pulse peak outputs in the range from 50 kW to 1 MW to be achieved.
Before or after the forming of the funnel, a cross-sectionally substantially circular or cylindrical (aperture) portion of the aperture can be formed by laser drilling, so that the aperture comprises a non-cylindrically formed funnel and a substantially cylindrically formed (aperture) portion. During the laser drilling of the cylindrical (aperture) portion, the laser beam is aligned in the direction of the center axis of the aperture, generally running at an acute angle in relation to the surface of the component.
During the removal of the layer, the laser beam can move spirally over the respective surface of the component, it being possible for the relative movement to begin at the center, where the center axis of the aperture intersects the outer surface of the component, or at the outer circumference of the funnel.
Further exemplary embodiments of the invention are described.